The present invention relates generally to conductivity instruments and, more particularly, is directed to apparatus for calibrating a conductivity instrument used to measure the conductivity of pure water.
In many instances, it is necessary to utilize high purity water. In order to ensure that the water is of sufficient purity, conductivity instruments are provided for measuring the conductivity of the high purity water. These instruments are extremely sensitive, producing conductivity measurements having a sensitivity of more than one part in sixteen thousand, that is, at least 10.sup.-4. It therefore becomes extremely important that the conductivity cell of the calibrating instrument be calibrated prior to such conductivity measurements.
The greatest problem with the measurement of conductivity is the determination of the cell constant. Even if the cell constant is determined accurately when manufactured, it can only reliably be calibrated to within 0.5-1.0%. When placed in use, the value of the cell constant can change for various reasons, including the proximity of other metals, and usage. Further, the surfaces of the electrodes of the conductivity cell become contaminated after use and increase the cell constant.
Various systems have been devised for calibrating or standardizing test cells of a monitoring system. For example, U.S. Pat. No. 4,151,255, discloses a method for standardizing test cells of a monitoring system. In normal operation, a sample is fed into a vessel which contains three electrodes. One electrode is a glass electrode for measuring the pH of the sample fluid contained within the vessel. Another electrode is a reference electrode for use in such measurement and the third electrode is a temperature sensing electrode which compensates for variations of temperature of the sample fluid fed to the vessel.
In order to assure that the monitoring and measuring analyzer provides an accurate measurement of the pH of the fluid sample from the process, a buffer solution of known pH is fed to the vessel of the test chamber. However, this patent requires two buffer storage means for holding two different standard buffer solutions for use in standarizing the reading of the pH measuring means during intermittant buffer standardization. The storage of different buffer solutions, and the necessary supply lines and valves associated therewith, makes this apparatus relatively expensive and complicated.
U.S. Pat. No. 4,445,091 discloses a system for determining the pH value of deionized cooling water by conductivity measurements, which includes a main loop and at least one parallel loop which includes an ion exchanger so that the two conductivity cells can be utilized by different measurement to provide a true measurement of the deionized water in the system. These ion exchangers fix the standard for one conductivity cell so that the measured difference between the two conductivity cells will correspond to the actual pH value of the deionized water in the system being measured. This patent, however, does not supply the calibrated water and water to be measured to the same conductivity cell. Rather, two different conductivity cells are used and an adjustment must be made based on calibration of only one of the cells, thereby making this system relatively complicated, and expensive, because of the additional conductivity cell that is required.
U.S. Pat. No. 4,357,143 shows a parallel flow system in which one portion of the sample water is passed through an ion exchange resin column and then the two samples are passed to a detection cell which has two ion specific electrodes therein connected through conventional bridge circuitry. The differential between the two cells is measured for the system. Thus, the use of two cells in this patent is similar to that of U.S. Pat. No. 4,445,091.
Attention is also directed to U.S. Pat. Nos. 1,684,645; 3,451,403; and 4,473,458.